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Reducing Cooling Drag on a Stemme S10

Reducing Cooling Drag on a Stemme S10

Reducing Cooling Drag on a Stemme S10

Hey Guys,

Im trying to reduce the cooling drag on a Stemme S10 by modifying the cooling ducts. However I am having trouble coming up with ideas. I was going to try to use a NACA duct initially but the surface of the airplane where the engine is located is at a negative slope to the airflow. If anyone has any ideas on how I would be able to use a NACA duct or any other ideas on cooling the engine, it would be much appreciated. I have enclosed a picture for reference.


RE: Reducing Cooling Drag on a Stemme S10

Naras 781,
Unless you are having problems with the engine overheating, I would leave it alone. The cooling ducts retract when the engine is shut off and the propellor is retracted.
  They stick way out into the airflow, because they are, sited in a turbulent region.

RE: Reducing Cooling Drag on a Stemme S10

What airspeed, or better, what Reynold's no. do the ducts operate at.  They might, but not likely, benefit from a b.l. fence/offset geometry.

RE: Reducing Cooling Drag on a Stemme S10

The ducts are open when ever the nosecone is extended and the propeller is being used. The cooling ducts double the drag on the Stemme when they are open. I have been tasked to find a way to reduce this drag. Any more help would be much appreciated.

RE: Reducing Cooling Drag on a Stemme S10

Also, the propeller blades extend into the airstream automatically by centrifugal force when the engine is started

RE: Reducing Cooling Drag on a Stemme S10


Look at first diagram, shows an offset inlet duct, the idea is to move the inlet out of the turbulent boundary layer.  The result is more flow (more total pressure) per square inch of protruding cross section, i.e. you can reduce the duct area quite a bit because the intake sees very little retarded flow from the boundary layer, and thus can lower drag.  P-51 Mustang used similar ducting, which (due to heat addition and good exhaust flow ducting of the radiator stream) actually added net thrust to the airframe.

RE: Reducing Cooling Drag on a Stemme S10

Thanks a lot btrueblood. Ill try it out and keep the thread updated on progress.

RE: Reducing Cooling Drag on a Stemme S10

Hi naras781

I agree with berkshire...which is the object to modify the inlets and exits of refrigeration system ? these are open
only whe you are going to start up the engine. In this condition the saving is immaterial on cooling drag since
the engine delivers necessary energy to fly.

It justifies an extensive modification of the system for any small saving ?

The Rotax used in Stemme is a turbocharged engine. I think is mandatory mandatory to assure a correct refrigeration to the turbo .Also to the intercooler, oil cooler,radiator, etc).

Best Regards

RE: Reducing Cooling Drag on a Stemme S10


The cooling systems that you showed me are for when the engine is in the front of the fuselage. Would this work for an engine behind the cockpit? I'm not sure where all the airflow is turbulent and what effect the prop has on the air.

RE: Reducing Cooling Drag on a Stemme S10

With engine on and prop spinning, I think it'd be safe to assume turbulent flow (or at least "disturbed flow") just about anywhere along the fuselage.

In reality, do a flat plate b.l. calculation for the speed of interest, and find the 90% momentum thickness, use that value for the inner edge of the intake ramp.  

I'd be taking total pressure profiles across the current inlets, and possibly at a few outboard stations as well, to guide the effort.  Data beats calculations every time.

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